The present invention relates to distortion or error canceling amplifiers. More particularly, the invention relates to a distortion-canceling amplifier that implements unequal power combining of signals.
As is known, spectral regrowth refers to the amplification of signals outside a desired frequency range. Ideally, an amplifier would amplify signals without creating noise, particularly noise outside the frequency range of the input signal. In practice, this ideal has not yet been achieved, and spectral regrowth often causes interference between adjacent communication channels. Limiting or reducing spectral regrowth is an important factor to improving spectral efficiency. When spectral regrowth is low, interference is reduced. With reduced interference, channel separation may be narrowed and the number of channels in a given bandwidth may be increased.
Feed-forward amplifiers (xe2x80x9cFFAsxe2x80x9d) use two amplifiers: a main amplifier and a distortion-canceling amplifier. The main amplifier is operated at a relatively high power level and generates an amplified, but distorted or noisy signal. A feed-forward circuit or path is used to estimate the distortion generated by the main amplifier. The estimated distortion is inverted, amplified, and then summed with the output from the main amplifier to remove the distortion in the amplified signal.
A balanced error correction (xe2x80x9cBECxe2x80x9d) amplifier also uses two amplifiers. In a BEC amplifier, the output power from a main amplifier and an error-canceling amplifier are combined in an equal-power combiner. This results in about a fifty-percent efficiency improvement over feed-forward amplifiers. Nevertheless, the performance of BEC amplifiers is less than ideal.
The equal-power combining technique used in a BEC amplifier has at least two efficiency limitations. First, the output coupler or combiner dissipates desired signal power due to the fact that the input signals applied to the combiner can never be exactly equal. The dissipation reduces efficiency. Second, the error-canceling amplifier is necessarily operated at a non-optimal point that requires relatively high levels of the distortion-canceling signal. This higher-than-optimal distortion canceling signal results in further distortion due to a non-linear transfer of the distortion-canceling signal. The error amplifier experiences a reduced canceling capability that can only be compensated by a reduction in the desired output signal power. This, in turn, reduces efficiency.
Accordingly, there is a need for an improved amplifier that eliminates the deficiencies in BEC amplifiers.
The invention provides an amplifier for communications and other applications in the form of an optimal power combining (xe2x80x9cOPCxe2x80x9d) amplifier. The OPC amplifier includes a main amplifier operable to receive a main signal and to generate an amplified signal having a main component and an error component. The amplifier also includes an error amplifier coupled in a feed forward arrangement to the main amplifier. The second amplifier receives an input signal and generates an output signal having a main component and an error component. A balancing network is coupled to the main amplifier and to the second amplifier. The balancing network isolates a sample of the error component of the output signal of the main amplifier, inverts the sample and combines the sample with the input signal to the error amplifier. An output combiner combines the output signal from the main amplifier and the output signal of the error amplifier in an unequal fashion such that the error components of the two signals substantially cancel one another.
The design of the OPC amplifier reduces losses due to the improvement in efficiency caused by appropriately accounting for unequal signal powers in the input signals of the combiner. Further, the design permits the error-canceling amplifier to be operated in a more linear fashion.
As is apparent from the above, it is an advantage of the present invention to provide an amplifier with improved power efficiency. Other features and advantages of the present invention will become apparent by consideration of the detailed description and accompanying drawings.